DE 199 03 439 A1 describes a procedure for operating a combustion engine, in whose exhaust duct a SCR-catalyzer (selective catalytic converter) is arranged, which reduces the nitrous gases that are in the exhaust gas of the combustion engine with a reagent to nitrogen. The metering of the reagent or the precursor of a reagent preferably takes place depending on operating parameters of the combustion engine, such as the engine speed and the injected fuel amount. Furthermore the metering preferably takes place depending on at least one exhaust gas parameter, as for example the exhaust gas temperature or the temperature of the SCR-catalyzer. The reducing agent ammoniac is used as the reagent, which is won from a reagent precursor, in particular a urea-water solution.
The metering of the reagent or the precursor of the reagent into the exhaust duct has to be determined carefully. A too low metering causes that nitrous gases cannot be completely reduced in the SCR-catalyzer anymore. A too high metering causes a reagent slip, which can cause an unnecessarily high reagent consumption on the one hand and depending on the quality of the reagent an undesired odor nuisance on the other hand.
DE 10 2004 031 624 A1 describes a procedure for metering a reagent into the exhaust duct of a combustion engine, at which a control or regulation of the reagent filling level in the SCR-catalyzer is provided on to a default reagent filling level. The aimed default of the reagent filling level ensures on the one hand that a sufficient amount of reagent is available in unsteady states of the combustion engine for a NOx-reduction as completed as possible and that one the other hand a reagent slip downstream after the SCR-catalyzer is avoided.
DE 10 2005 042 487 A1 describes a procedure for metering a reagent or a precursor of a reagent into an exhaust duct of a combustion engine, which also contains a SCR-catalyzer. The reagent filling level in the SCR-catalyzer is here also controlled or regulated on to a default reagent filling level, but determined on to a maximum value, which corresponds with a SCR-catalyzer that is completely filled with reagent. If sensor signal changes occur a too high metering can always be assumed. The SCR-catalyzer is always operated with the maximally possible efficiency, at which the maximally possible NOx-conversion occurs.
DE 10 2005 042 489 A1 describes a procedure for operating a combustion engine, in whose exhaust duct at least one SCR-catalyzer is arranged, which is impinged with a reagent, which contributes to the NOx-conversion. An amount for the NOx-concentration that appears downstream after the SCR-catalyzer is calculated and also measured with a NOx-sensor, which provides a lateral sensitivity towards the reagent. A difference between the calculated amount and the measured amount of the NOx-concentration downstream after the SCR-catalyzer is determined, which is considered at the determination of the metering rate of the reagent or the precursor of the reagent. Provided is a reasonability check, at which the amount of reagent or precursor of a reagent that is metered in a length of time and the reagent amount that is converted in the SCR-catalyzer and/or the converted NOx-amounts are compared to each other. In the case of a deviation a correcting signal is provided, which adapts the metering signal.
An improvement of the way of proceeding that is described in DE 10 2005 042 489 A1 is stated in DE 10 2005 042 490 A1, which provides that the differences are determined in a timely order and saved. The metering rate is determined depending on a default amount and/or default order of evaluation results of the differences. Thereby a long-term adaptation of the metering rate is achieved.
DE 10 2005 042 488 A1 described a procedure for operating a combustion engine, in whose exhaust duct a SCR-catalyzer is also arranged, which is impinged with a reagent or a precursor of a reagent, which contribute(s) to the NOx-conversion in the SCR-catalyzer. Calculated is an amount for the NOx-concentration that occurs downstream after the SCR-catalyzer. A NOx-sensor, which is arranged downstream after the SCR-catalyzer, provides a sensor signal, which corresponds at least with the NOx-concentration and due to the lateral sensitivity of the NOx-sensor with a reagent slip. A reagent signal, which determines the metering rate of the reagent, is influenced by a correcting signal depending on the difference between the measured and the calculated NOx-concentration each downstream after the SCR-catalyzer and depending on a measure for the temperature of the SCR-catalyzer.
The invention is based on the task to provide a procedure for metering a reagent into the exhaust duct of a combustion engine and a device for implementing the procedure, which allow an adaptation of the metering rate, whereby a continuous NOx-conversion is ensured.
This task is solved by the characteristics that are stated in the independent claims.